ACCRA AND BEIJING—Dan Peng steps onto a narrow steel frame just above a 6-meter-deep pool, which holds a nuclear reactor about to go critical. Clad in a lab coat with a pocket radiation dosimeter, sweating in the stifling reactor hall here on the outskirts of Ghana’s capital, the young nuclear physicist edges out to a tube jutting above the water’s surface. He grabs a cord leading out of the tube and reels it up, hand over hand, until a cigar-shaped capsule emerges—a packet of neutron-absorbing cadmium. It’s the last of three that were immersed in the pool as a safety measure, to ensure that the reactor’s new low-enriched uranium (LEU) core did not achieve a self-sustaining fission reaction—criticality—before the team was ready.
At the edge of the pool, several other physicists and engineers, colleagues of Peng’s at the China Institute of Atomic Energy (CIAE) in Beijing, huddle behind a bank of …
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